So are you saying that bash is not actually executed but the characters are being printed on the screen and they looks as if the animation were still playing?

What I cannot understand from that explanation is how the animation time works. It should display the animation in chunks, with really fast frames, followed by pauses while it waits for data to be received.

Without downloading it (on my phone), I expect there are ANSI codes involved to move the cursor and control other aspects of the terminal:

https://en.wikipedia.org/wiki/ANSI_escape_code?wprov=sfla1

curl always goes to stdout if you don't redirect it. i'd guess the file is just padded so that the "animations" take longer.